Gas detector and analyzer



Feb. 2, 1965 H. M. CRAWFORD 3,167,947

GAS DETECTOR AND ANALYZER Filed Aug. 7. 1961 (FIXED VOLUME) SAMPLE |N WTSAMPLE OUT Harry M. Crawford Inventor By M z Patent Attorney UnitedStates Patent ()fitice 3,167,947 Patented Feb. 2, 1965 3,167,947 GASDETECTQR AND ANALYZER Harry ,M. Crawford, Point Murray, N1, assignor toEsso Research and Engineering Company, a corporation oi Delaware FiledAug. 7, E961, Ser. No. 129,659 9 Claims. (Cl. 73--23.ll)

This invention relates to a new detection and measurement technique.More specifically, this invention teaches the application ofdifierential absorption or solubility phenomenon to separate, detect andmeasure one or more components of a gaseous mixture.

The measurement and detection of one or more trace components in gaseousmixtures is of great importance in the chemical and petroleumindustries. It is particularly necessary to be able to rapidly detectand measure the concentration of components in samples so as to maintainproduct quality and make proper adjustments in processes before harmfulresults occur.

The instant invention has many advantages over hitherto used detectionapparatus. For example, when gas chromatography is used to detectmoisture, there is a definite lack of sensitivity when measuring lowmoisture contents in the parts per million range. In addition, bottledgas is needed to act as a carrier and generally the apparatus is quitecostly, ranging from four to six thousand dollars for plant-typeequipment. Electrolytic type moisture analyzers also sufier fromdisadvantages, namely, they foul when unsaturated hydrocarbons arepresent, recombination errors result, there are sever range limitations,and they have a high service factor since the cells are readily fouledwith foreign matter. Conventional adsorption moisture analyzers have adisadvantage of being of extremely high cost, that is, in the range ofnine thousand dollars. Additionally, they create polymerization problemswith unsaturates and require complicated readout systems.

In accordance with the instant invention, it has been found that byapplying diiterential adsorption and/or solubility phenomenon, one 'ormore trace components of a gaseous mixtue may be readily detected andmeasured. Briefly, this technique involve the use of the pressurecycling of a sample so that it may serve to regenerate the detectingmeans. Where adsorption phenomenon is used, advantage is taken of theheat of adsorption liberated when one "or more components of a gasmixture stream are selectively adsorbed on an adsorbent. Since thetemperature rise is directly proportional to the amount of heat adsorbedand since the heat of adsorption of particular materials on particularadsorbents is well known, it is possible to correlate the temperaturerise with the amount of material present in a fixed quantity gas. Inorder to regenerate the adsorbent, the absorption eflluent is backflowedover the adsorbent material at a reduced pressure. By so doing, theadsorbed material is purged from the adsorbent, thereby readying theadsorbent for subsequent use.

To more fully illustrate the instant invention, attention is directed tothe attached figure. An adsorbent bed E is located within an adsorptionzone and provided with a sample inlet conduit wherein is located valve Band a sample outlet conduit wherein is located valve A. The far end ofthe adsorbent bed E is connected to fixed volume trap G. Intermediate tothe adsorbent bed E and the fixed volume trap G is valve F. ThermistorsC and D are located respectively in the forward and rear ends of thebed. They are connected to a bridge circuit and indicator or recorder(not shown). In the operation of the analyzer, a sample containing moistair is admitted through valve B into the adsorbent bed B. At this time,

valve A is closed. The sample is allowed to flow across the adsorbentbed into the fixed volume trap G for a fixed time or until the pressurein the fixed volume trap equals the pressure of the regulated sample.Only a predetermined amount of the gaseous material will flow across theadsorbent bed. During the flow of gas into the fixed volume trap, thepressure at E is above atmospheric because of the constriction createdby the partially opened valve F. .As the sample passes through theadsorbent bed E, moisture is adsorbed primarily in the vicinity of thethermistor C. This results in the evolution of heat. The adsorbent bed Eis selected so as to be of a sutficient size so that moisture does notbreak through to the vicinity of D. Thus, the thermistor D serves as areference thermistor. The differential heating which results, as notedabove, is measured by a conventional bridge circuit. After thetemperature differential measurement is made, valve B is closed andvalve A opened to a lower or atmospheric pressure region. Gas from thefixed volume trap G then flows in a reverse direction over the bed E ata lower pressure. The moisture previously adsorbed in the vicinity ofthermistor C is now totally desorbed. This desorption results in thecooling of the desiccant in the vicinity of C and substantially restoresthe adsorbent bed to a uniform temperature. Both the heats of adsorptionand desorption may be related directly to the moisture content of thesample.

While the above description employs an adsorbent bed to detect themoisture, it is equally within the scope of this invention to utilize aliquid substrate which will partition one or more of the components ofthe gaseous mixture. The heat of solution may be measured in a manneranalogous to that described above. The adsorbed which is used will, ofcourse, depend upon the component or components to be analyzed. Forexample, where it is desired to measure moisture, an adsorbent such assilica gel can be advantageously used.

Other examples of samples which may be analyzed include moisture insaturated and unsaturated hydrocarbons and inert gas streams with Dowexion exchange resin, the detection of carbon dioxide with activatedalumina, and the detection of oxygen with molecular sieves. Hydrocarbonsin hydrogen may also be detected by using activated alumina or charcoalas the adsorber. In fractionating towers, the overhead product may beanalyzed by using 5A molecular sieves or alumina to detect normalbutane.

The size of the adsorbent bed or substrate must be selected so that noneof the adsorbed component appears in the far end of the bed.Determination of the geometric shape and weight of adsorbent is shown bythe following sample calculation:

(1) Select geometry of trap. For convenience, trap should be kept smallas possible so as to make the whole apparatus compact and reduce load ofthermostatcd models.

(2) Select operating pressure. Low pressure is advantageous since itallows the measurement of high dew point gases.

(3) Calculate volume of gas to be dried=volume at conditions of sample(operating pressure)=volume at reduced pressure.

(4) For given moisture range calculate weight of desiccant required foradsorption.

Example.Activated alumina can adsorb water at 100% efiiciency untilweight of Water adsorbed equals 12% of the weight of alumina.

(5) From (3) and (4) weight of desiccant can be calculated usingpublished data and following simple procedures used in sizing commercialdriers.

(6) Bed geometry should be such as to promote pisthough no moisture ispresent.

employed in certain applications.

or prior to C as the gas passes into G. The amount of desiccantcalculated in step should be placed in bed 6 prior to C. p

(8) Reference detector D can be placed deeper in the same bed. Adistance twice that of C gives a safety factor of 2. iHowever, thisdistance should be kept small since the bed between C and D hassomefinitev heat ca-: pacity. Heat capacity between C and D can giveraised temperature difierentials inun thermostated models even can belocated out of the bed and prior to C if this is found to present aproblem.

The adsorption pressure is determined by the pressure atwhich the sampleis'introduced and by theconstriction of the valve between the adsorbentbed and the fixed volume trap.

In normal operation valve F fice. pressure during the adsorption cycle(Le. during the flow of gas into G). The. desiccant, therefore; comes toequilibrium with the adsorbed water at a positive pre sure. Also at thiselevated pressure the piston flowof a moisture front is less than agiven volumeof gas at standard temperature and pressure. 1 e

During the purge-portion of the cycle the fixed orifice F reduces thepressure of the gas in G and nowan is utilized as afixed ori- Itspurpose is to hold the bed above atmospheric moconductivity instruments,and other detectors known in the field of gas chromatography may beemployed in the instant invention. I v

To more fully illustrate the instant invention, the following example isset forth.

It is conceivable'that D Example 1 7 Using the apparatus similar to thatshown in'the attached figure, samples of air with known moisture contentwere analyzed. .The adsorbent, Dowex ion'exchange resin, was placed in aspring loaded adsorption zone having an inside diameter of about 0.34inch and a length of 3 inches. Matched thermistors (VECO #Al56) werelocated 1 inch and 1 /2 inches from the sample inlet end of theadsorbent bed. Thesethermistors, were wired to a conventional bridgecircuit which,

in turn, was connectedto a recorder having a range of from l.25 to +1.25millivolts.

The fixed volume trap was made up of 90 inches ofv inch O.D. coppertubing. A precision needle valve (NUPRO) served as value F and athree-way air-operated valve (Humphrey model environment (reducedpressure) is created in which the desiccant must give up the previouslyadsorbed moisture to reach equilibrium.' At this reduced pressure, thepiston flow is greater due to the expansion of the gas from G. i

Although a fixed orifice can be used, the use of a needle valve isconvenient since it can be easily adjusted (measure rise time ofpressure in G with stop Watch and pressure gauge). Also, for a givencycle time, the amount of sample (hence, the range of the instrument)can be changed if needed by simple adjustment of the valve. I

The desorption pressure, that is, the pressure over the adsorbent duringthe backfiowing part of the cycle, is preferably atmospheric. This isconvenient since it permits simple venting of the sample. However, undersome circumstances, it might be desirable to desorb at subatmosphericpressures. analyzed liquefies at high pressures, it may be necessary touse lower adsorption pressures and to desorb under a vacuum. Inaddition, when there is a large amount of adsorbed component, sayQover20%, vacuum desorption may be necessary to completely regenerate theadsorbent.

Many modifications of the instant invention may be made withoutdeparting from its spirit. For example, instead of using thermistors,pressure gaugesrnay-be a particular component will reduce the amount ofmaterial in the gas phase, the pressure in the chamber may be correlatedwith the amount of adsorption. When this technique is used, a fixedvolume of sample is passed over the bed. This fixed volume is preferablytrapped at a positively controlled pressure before passage over theadsorbent'bed. Where it is desirable to detect'more' than one component,different adsorbents may be used "in tandem. For example, Dowex may beused to detect water, followed by activated alumina for the detection ofcarbon dioxide, followed by the. use or" molecular sieves for thedetection of oxygen. In this case,. the

amount of sample may be conveniently regulated by determining 'thepressure in the trap. Basically, the same flow scheme as described'inthe above figure would be employed. The main difierence would be in thelength of the column which, like in chromatography,

ing of the sample.

response was linear.

125A) served to control the introductionland exhaust- Thesample inletpressure was 8 p.s.i.g. and the exhaustpressure was atmospheric.- Cycletime was regulatedby an electrical timer (Industrial Timer Corporation;Model CM-O) to 24 cycles per minute. Using air with known moisturecontent, it was found that there was 1.0 millivolt deflection for each100 ppm. of water by volumeand, furthermore, the

What is claimed is:

l. An improved autoregenerating gas analyzer which comprises: anadsorption zone containing adsorbent material; a sample inlet meansconnected to one end of said adsorption zone; a fixed volume trap; aconduit connecting said trap to the other end of saidv adsorption zone;a pressure reducing meansin said conduit; asample outlet means connectedto said one end of said adsorpt on zone; valve means for permitting agaseous sample Since adsorption of to alternately flow into saidsampleinlet means and outwardly through said sample outlet means;temperature sensitive means in both said ends of said adsorption zonefor detecting a temperature differential between said one end and saidother end of said adsorption zone.

2. The improved gas analyzer of claim 1 wherein said pressure reducingmeans is a valve.

3. The improved. gas analyzer of claim 1 wherein said pressure reducingmeans is an orifice.

4-. The improved gas analyzer of claim 1 wherein said temperaturesensitive means are thermistors.

5. The improved gas analyzer of claim 1 wherein the "said adsorbentmaterial is an ion exchange resin.

6. An improved process for analyzing a key compo nent of a gaseousmixture which comprises: introducing said gaseous mixture into anadsorbent zone containing an adsorbent selective of said key componentin a, positive fiow direction at a relatively high pressure during anadsorption step, said adsorbent being initially substantially free ofsaid key component and comprised of two portions, a forward portion,said portion comprising the area in which the said key component istotally adsorbed, and a rear portion, the said rear portion comprisingthe rest of the said adsorbent zone, adsorbing said key component on thesaid forward portion of said adsorbent; discontinuing the introductionof said gaseous material prior to the absorption of said key componenton the said rear portion of said adsorbent, concurrently detecting thetemperature differential between said front and said rear portions ofthe said adsorbent zone thereby determining the amount of said keycomponent present in said gaseous mixture, thereby terminating saidadsorption step; Withdrawing an effluent substantially free of said keycomponent from said adsorption zone; concurrently reducing the pressureof said effluent; collecting said reduced pressure efiiuent; reducingthe pressure in said adsorption zone and then backflowing therethroughsaid collected efiluent, thereby desorbing said key component from saidadsorbent; Withdrawing a second effluent from said adsorption zone, saidsecond effiuent having substantially the same composition as saidgaseous mixture.

7. The process of claim 6 wherein said key component is water and saidgaseous mixture is moist air.

8. The process of claim 6 wherein said adsorbent is an ion exchangeresin.

9. The process of claim 6 wherein said temperature differential isdetected by means of thermistors located in said forward and rearportions of said adsorbent.

Christensen May 10, 1960 Skarstrom Dec. 5, 1961

1. AN IMPROVED AUTOREGENERATING GAS ANALYZER WHICH COMPRISES: ANADSORPTION ZONE CONTIANING ADSORBENT MATERIAL; A SAMPLE INLET MEANSCONNECTED TO ONE END OF SAID ADSORPTION ZONE; A FIXED VOLUME TRAP; ACONDUIT CONNECTING SAID TRAP TO THE OTHER END OF SAID ABSORPTION ZONE; APRESSURE REDUCING MEANS IN SAID CONDUIT; A SAMPLE OUTLET MEANS CONNECTEDTO SAID ONE END OF SAID ADSORPTION ZONE; VALVE MEANS FOR PERMITTING AGASEOUS SAMPLE TO ALTERNATELY FLOW INTO SAID SAMPLE INLET MEANS ANDOUTWARDLY THROUGH SAID SAMPLE OUTLET MEANS; TEMPERATURE SENSITIVE MEANSIN BOTH SAID ENDS OF SAID ADSORPTION ZONE FOR DETECTING A TEMPERATUREDIFFERENTIAL BETWEEN SAID ONE END AND SAID OTHER END OF SAID ABSORPTIONZONE.